FIG. 1 is a view showing a meridian plane of a conventional volute pump, and FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1. As shown in FIGS. 1 and 2, a liquid, which has flowed through an inlet port 1 into an impeller 20, is given velocity energy by the rotation of the impeller 20, and is discharged in a circumferential direction into a volute-shaped flow passage 11 defined in a pump casing 10. The flow passage 11 is formed such that its cross-sectional area increases gradually as it approaches a downstream side. Because of this gradually-increasing cross-sectional area of the flow passage 11, the velocity of the liquid that is flowing downstream through the flow passage 11 is decreased while its velocity energy is converted into pressure energy. The liquid is discharged out through an outlet port 2.
The pump casing 10 includes a protrusion 12 located near a terminal end of the volute and projecting into the flow passage 11 that is in the shape of volute. This protrusion 12 separates a starting end of the volute from the terminal end of the volute. FIG. 3 is a view showing the protrusion 12 and the impeller 20 as viewed from a direction indicated by arrow A in FIG. 2. As shown in FIG. 3, a gap C is formed between the protrusion 12 and the impeller 20. The protrusion 12 has a distal edge that is formed by a curved surface whose cross section is represented by a circle of curvature (indicated by dotted lines in FIG. 3). This circle of curvature has a radius of curvature R that is constant throughout the protrusion 12 from one side end to the other side end of the protrusion 12. In FIG. 3, a dot-and-dash line represents a position of the center of the circle of curvature of the distal-edge cross section of the protrusion 12.
As shown in FIG. 2, the liquid that flows through the flow passage 11 is divided by the protrusion 12, whereby a part of the liquid passes through the gap C to circulate in the pump casing 10. In consideration of the pump efficiency, it is desirable that the radius of curvature of the cross section of the distal edge of the protrusion 12 be small in order for the protrusion 12 not to cause a disturbance of the flow of the liquid. Furthermore, the gap C between the protrusion 12 and the impeller 20 should desirably be small in order to reduce an amount of the circulating flow.
As shown in FIG. 3, when the velocity of the liquid in the pump casing 10 is high, i.e., when the flow rate of the liquid is high, most of the liquid, which has been introduced through the inlet port 1 into the impeller 20, flows along a main plate 20a of the impeller 20. When the velocity of the liquid in the pump casing 10 is low, i.e., when the flow rate of the liquid is low, most of the liquid flows along a side plate 20b that is opposite to the main plate 20a. Although FIG. 1 illustrates an example of a closed-type impeller which has the main plate 20a and the side plate 20b, the liquid flows in the same manner in an open-type impeller which is free of main and side plates and in a semi-open-type impeller which is free of a side plate.